With the demand for reduced size substation the gas-insulated substation components, such as circuit breakers, has been developed and in this type of component, the insulating gas requires attention as to its temperature especially in the two-pressure type of circuit breaker. In this type of arrangement, gas in a high pressure chamber discharges into a low pressure chamber in which the separable circuit interrupting contacts are housed. In such apparatus, the insulating gas in the high pressure chamber must be maintained above a predetermined temperature to prevent the insulating gas from liquefying in which state it loses its dielectric properties. To overcome this undesirable condition, various heating arrangements have been proposed.
In U.S. Pat. No. 2,955,182, a heater unit of the resistance type is indicated as being secured to the outside of the high pressure tank. In U.S. Pat. No. 3,118,995, a nonmagnetic strip is utilized to force magnetic flux to pass through the wall of the high pressure tank to set up eddy current loses within the tank wall and thereby heat the insulating gas. In U.S. Pat. No. 3,137,777, the heat of the compressor is utilized to heat the gas entering the low pressure side of the compressor. In U.S. Pat. No. 3,303,310, a resistance coil heater is shown wound around the insulated high pressure tank. In U.S. Pat. No. 3,358,104, there is disclosed an arrangement in which a current transformer is inductively coupled to one of the terminal bushings of the circuit breaker and utilized to provide current for a heating element in the high pressure tank. U.S. Pat. No. 3,359,390 discloses a filament winding heating arrangement in series with the compressor to heat the insulating gas. U.S. Pat. No. 3,566,062 indicates the desirability of using resistance heaters with the main gas reservoir. U.S. Pat. No. 3,846,601 discloses a method of heating the insulating gas by current flow through a hollow terminal stud which communicates with a high pressure gas tank. U.S. Pat. No. 3,903,388 utilizes a heater blanket around the high pressure gas tank to maintain the gas in its gaseous state.
All of the aforementioned U.S. patents indicate the on-going search for a more efficient and accurate means for heating insulating gas with accurate heating sensing. It is also apparent that the prior art, as exemplified by the aforementioned patents, have taken the approach of heating the tank or the tank line with temperature controls located very close to the heaters. This, it is believed, is not an efficient way of heating and, with temperature controls close to the heaters, they tend to shut the heaters off before the gas reaches a correct temperature.